Cell migration is one of the most basic physiological processes. Even though the physical environment has been shown to regulate cell migration, most research has focused on cells migrating on rigid, 2D surfaces. In nature, cells migrate in 3D environments where they must constantly negotiate the balance between cellular and extracellular forces and between cellular and extracellular volumes. A striking feature of 3D migration is the frequent occurrence of blebs, which are hemispherical protrusions devoid of actin. The roles of these blebs have been little investigated. In particular it is still unknown ho, or even if, blebs directly enable motility by exerting forces on their environment. We hypothesize that blebs promote 3D motility under conditions in which pseudopodial extensions, which lead migration in 2D, mechanically stall and thus fail to allow directed migration. To test this hypothesis, we will develop computational tools to characterize 3D cell morphology in an unbiased manner, and to find, track, and classify protrusive structures such as blebs. To distinguish protrusive types, we will also develop tools to measure the localization of cortex-associated proteins. Since the collagen matrix plays an active and integral role in 3D cell migration, we will further develop tools to extract the locations and properties of the individual collagen fibrils that compose the collagen matrix. Migration results from the mechanical interaction of cells with their environment. We will therefore track how blebs and other protrusive structures interact with and deform individual collagen fibrils, and we will measure the localization of proteins at bleb-fibril interfaces. We additionally hypothesize that blebs play different functions in different mechanical environments. We will test this hypothesis by searching for mechanical environments in which blebs transition between functional states. In particular, we will begin to analyze plasticity between modes of blebbing motility by testing the hypothesis that, as collagen stiffness is increased, blebbing gradually transitions from pushing fibrils away to interdigitating pores, and thus both functional bleb types can coexist in the same cell. Together, this work will refocus migration research on a model centered on cell-environment interactions.